1. Field of the Invention
The present invention relates to a structure of a high density multi-layered thin film circuit board for mounting electronic elements thereon, such as large scale integrated circuits (LSI) or input/output pins, particularly to a structure for obtaining an electrical conduction between front and back surfaces of such a multi-layered thin film circuit board.
2. Description of the Related Art
Recently, a thin film circuit board capable of providing high density circuits, a so-called MCM (multi-chip module) substrate, and applicable to an electronic equipment such as a computer, has been put into practical use. In the circuit board, the formation of an extremely thin dielectric layer of polyimide has become possible by means of a spin coating method.
In this case, since signal patterns are also formable by a sputtering method and an etching method using a high-density resist, it is possible to realize high density patterns more dense than those of an ordinary printed circuit board.
In the prior art, an MCM called an MCM-D/C (D is an abridgment of xe2x80x9cdepositxe2x80x9d and C is that of xe2x80x9cco-firingxe2x80x9d) has been developed, and put into practice, wherein thin film circuits are formed in a multi-layered manner on a ceramic substrate and thick film circuits are formed in the interior thereof. This is because it enables a number of terminals to be mounted thereon and a wire length necessary therefor to be shortened by mounting functional elements such as an LSI or the like on one surface and input/output terminals on the other surface, resulting in a significant advantage in the performance thereof.
In this regard, as a prior art, Japanese Unexamined Patent Publication No. 9-18144 discloses a multi-layered circuit board of glass ceramic wherein connector electrodes, capable of carrying input/output terminals on a glass ceramic member with sufficient strength and reliability, are provided on a back surface of the glass ceramic multi-layered circuit board. This circuit board has conductors of copper or copper alloy at least in the interior thereof used as connector terminals for arranging input/output terminals on the back surface and thick film pads of copper or copper alloy embedded in a back surface and connected to the conductors.
The thick film ceramic substrate formed by the co-firing method, however, has drawbacks of high production cost, which is particularly true of that having no signal circuits in the respective layer but only having vias bored between front and back surfaces thereof. Accordingly, a substrate is required which has a structure low in production cost and capable of achieving a desired function.
Accordingly, an object of the present invention is to provide a high density thin film circuit board and a process for producing the same, which board is capable of providing electrical conduction between front and back surfaces of the multi-layered thin film circuit board such as an MCM or the like at a low cost.
To achieve the above-mentioned object, according to the present invention, a high density thin film circuit board is provided, having at least one thin film circuit layer on a first surface of a substrate, wherein a conductor layer is disposed in the lowermost layer of the thin film circuit layer in contact with the first surface of the substrate, characterized in that holes for providing electrical conduction between front and back surfaces of the substrate are formed through the substrate from the first surface to a second surface thereof so that the conductor layer is exposed in the hole, wherein the diameter of the hole is gradually enlarged from the first surface to the second surface.
Advantageously, a solder bump to be in contact with the conductor layer is formed in the interior of the hole. In this case, the inner surface of the hole is preferably provided with an Ni-plated layer for preventing the solder from diffusing and an Au-plated layer for improving the wettability to solder.
Favorably, a metallic input/output pad is formed on the inner surface of the hole. In this case, a solder bump is preferably formed on the input/output pad. Also, an input/output pin is favorably mounted onto the input/output pad.
Advantageously, etchable glass, metal or Si wafer is used as the substrate.
Further, according to the present invention, a process for producing a high density thin film circuit board is provided, having at least one thin film circuit layer on a first surface of a substrate, wherein a conductor layer is disposed in the lowermost layer of the thin film circuit layer in contact with the first surface of the substrate, characterized in that the process comprises the steps of: forming the at least one thin film circuit layer on the first surface of the substrate, and etching the substrate from a second surface thereof to form a hole so that the conductor layer is exposed in the hole.
Preferably, the process further comprises a step for forming a solder bump by adhering solder to a metallic surface of the conductor layer exposed in the bottom of the hole and heating the solder until it is melted. In this case, the solder may be supplied to the interior of the hole by a printing, or the solder may be supplied to the interior of the hole as a solder ball.
Preferably, prior to supplying the solder, Ni/Au-plated layers may be formed on the inner surface of the hole for the purpose of an improvement in wettability to solder.
Or, the process may comprise a step of forming a metallic input/output pad on the inner surface of the hole. In this case, the process may further comprise a step for mounting an input/output pin onto the input/output pad.
Favorably, prior to forming the hole by etching, a guide hole is formed in advance, by the use of a laser beam, and is thereafter enlarged by the etching operation to complete the hole.
As described above, according to the present invention, an MCM can be provided which is capable of mounting electronic elements on opposite surfaces of a substrate without using a ceramic substrate having vias for conducting front and back surfaces thereof as in the prior art and which allows more inexpensive materials to be employed for this purpose.